I liked vog more then crota and crota more then oryx or skolas. That said adding an extra difficulty would also change things up since we have been overpowered for a long time. Not many people would be used to being underleveled for those raids and it would be a nice little kick in the rear to force players to take those raids seriously. It would also offer another reason for players to get familiar with raid mechanics and the level of team work needed for the raid.